Other comcom-ponents—including landfill gas collection and disposal, leachate collection and treatment, hydraulic control of groundwater, and remediation of contaminated groundwater and
Trang 1with Covers
This chapter describes the application of covers (also called caps) as com-ponents of landfill remediation Other comcom-ponents—including landfill gas collection and disposal, leachate collection and treatment, hydraulic control
of groundwater, and remediation of contaminated groundwater and surface water—are discussed at length by numerous authors (e.g., US EPA 1991; Dunn and Singh 1995; McBean et al 1995; Koerner and Daniel 1997; Gill et al 1999; Weand et al 1999)
Landfill covers are used at various times during a site’s active life At modern landfills, a thin soil layer or other cover is placed over the waste at the end of each day to control odors, prevent litter movement by wind, and keep rodents, birds, and insects out of the waste Intermediate soil covers protect inactive areas of an active landfill McBean et al (1995) present a more complete dis-cussion of daily and intermediate landfill covers
Landfill remediation includes a final cover; it remains in place as a part of the containment system This book focuses on final covers; they are the most frequently required, and are often the most complex and costly component of landfill remediation In the context of remediation within this book, the word
cover means a final landfill cover
This chapter contains a review of requirements for remediation, risk-based remediation, the site-specific environment, conventional and alternative cov-ers, and cover selection
2.1 requirements for lanDfill Covers
There are fundamental scientific and technical reasons for placing a cover on a land-fill Regulations control the selection and design of landfill covers; however, they are based on specific environmental concerns and have a technical basis Landfill covers provide several environmental benefits, but they have three primary goals:
Minimize infiltration into the waste and percolation from the waste to
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groundwater
Isolate the wastes from receptors and control their movement by wind and
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water
Control landfill gases
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Trang 2These three goals are common to all landfill cover designs; their implementation may include conventional covers based on regulatory requirements However, alter-native landfill covers also satisfy these goals and may provide a more protective, longer-lasting, and less costly solution
Landfill covers are intended to remain in place and protect the environment for an extended period, perhaps centuries; therefore, they should be durable and self-renewing Landfill covers should satisfactorily control infiltration of precipita-tion into the waste because it has potential to carry soluble wastes downward to groundwater Covers that meet the infiltration requirement usually satisfy the second requirement, that is, that the waste should be isolated from receptors and its move-ment controlled
Gas collection may be required to dispose of explosive and toxic landfill gas generated by the biodegradation of organic matter and other chemicals in the waste This is especially true for landfills with covers that include barrier layers because they trap and accumulate gas; thus, they usually need gas collection and disposal systems The long-term operation and maintenance of an active or passive gas col-lection and disposal system, if required, are significant financial burdens for the landfill owner
The migration of landfill leachate into an aquifer is important because it may cause significant groundwater contamination and the need for expensive remedia-tion However, recent work demonstrates that natural attenuation may control the extent of groundwater contamination caused by some contaminants Leachate from
a landfill that enters the groundwater contains organic material; it, in turn, produces anaerobic conditions in the groundwater under and down gradient from the landfill The anaerobic groundwater conditions degraded important contaminants Therefore, controlled leaching of landfill waste may be beneficial in some cases (Hicks et al 2002), altering the requirement to minimize infiltration to groundwater In any case,
it is necessary to control leachate to meet site requirements
2.2 risk-BaseD/performanCe-BaseD remeDiation
Previously, regulatory preference for use of design parameters contained in regula-tions limited or precluded the application of alternative landfill covers and designs for landfill remediation Currently, the regulatory control of landfill covers is changing
to allow consideration of alternative technologies Risk-Based/Performance-Based (RB/PB) evaluation of landfills is a process that applies engineering and science to the selection among remediation alternatives and allows better decisions There is already a strong regulatory basis for this process, and it is in use for other types of remediation efforts (Gill et al 1999)
An RB/PB landfill evaluation is a technical approach to selection of protec-tive remedial options based on the specific conditions at a landfill Using an RB/PB evaluation allows the landfill owner to determine the technical performance require-ments for a cover at a particular site
Trang 3The RB/PB landfill evaluation process follows four well-defined steps:
1 Identify releases: On the basis of known waste materials and environmental sampling, identify the actual and potential releases associated with a par-ticular landfill, including
Surface materials
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Gas generation
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Leachate production
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Groundwater and surface water contamination
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2 Assess exposure: Determine the exposure pathways to potential receptors, and whether the pathway is complete for each actual or potential release, including Direct contact
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Airborne contamination
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Surface or groundwater contamination
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3 Assess risk: Estimate the risks associated with each completed source– pathway–receptor combination
4 Establish site-specific performance requirements: Determine the specific performance requirements for each action needed to address the risks iden-tified, including
Cover requirements to eliminate direct contact
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Required control of infiltration to adequately control risks from
poten-•
tial leachate
Collection and treatment of gas, if necessary
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Control of groundwater contamination
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No further action if no significant risks were identified
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The landfill owner may use any landfill remediation method, including alternative covers, which meets the performance requirements after they are fully accepted This process allows the owner to select the most technically sound and cost-effective landfill remediation for a particular landfill
2.3 faCtors that influenCe remeDiation
Both selection of cover type and its design are dependent on specific site character-istics Site characteristics that have a dominant influence on covers include climate, soils and plants, landfill characteristics, hydrogeology, gas production, seismic envi-ronment, and reuse of landfill areas
2.3.1 c lImate
Precipitation (rain, snow, and sleet), solar radiation, air temperature, wind, and rela-tive humidity are the main climatic factors that affect landfill covers Precipitation amount and distribution in time has a direct bearing on infiltration of water into the cover and, potentially, into the buried waste Climatic factors influence ET, which controls soil water content and percolation through the cover soil Climate may also influence moisture content and temperature of the waste, which in turn controls
Trang 4waste degradation rate Climatic factors that control soil erosion include precipita-tion amount and intensity, as well as wind
The commonly reported annual or monthly averages of climatic variables do not provide sufficient information with which to evaluate a site Daily and seasonal climatic variation controls daily amounts of deep percolation into the waste For example, if the majority of precipitation falls during the season when vegetation is dormant, the potential for infiltration through the cover is greater than if the pre-cipitation falls during seasons of active plant growth A rainy day following a rainy day is more likely to produce water movement through the cover than a rainy day following a dry day
There is a strong influence from daily or even hourly climatic patterns, for example, Precipitation during one or two cloudy and cool days may result in greater
•
infiltration potential than the same total amount of precipitation spread over several days with periods of ET interspersed between the rain events
A single, relatively small rainfall event during or immediately following
snow-•
melt when vegetation is dormant has the potential to cause deep percolation
2.3.2 l andfIll and W aSte c haracterIStIcS
The operating history, wastes, and physical construction of the landfill all affect the remediation options that may be used For example, some of the characteristics that affect cover design include the type of waste deposited, whether or not the landfill has a liner, the age of the landfill, whether the landfill is active or inactive, and the amount of leachate produced by the waste
The type of wastes disposed in a landfill leads to its classification as (1) municipal (consisting of typical household wastes), (2) hazardous, (3) radioactive, or (4) mixed waste (nonradioactive mixed with radioactive) The waste classification directly affects the cover design because of both the technical and the regulatory requirements
As a landfill ages, the degradation of the waste and the pressure of overlying materials lead to compression and settling of the waste, sometimes by as much as 33% (Suter et al 1993; Sharma and Anirban 2007) Landfill subsidence is likely to
be severe for landfills containing deep deposits of fresh waste The resulting subsid-ence of the overlying cover can cause cracks in clay barriers, separation of geomem-branes (GMs), and slope changes that adversely affect surface water drainage and erosion Landfills that are old, when covered, are less likely to experience excessive surface subsidence
2.3.3 h ydrogeology
The distance between the bottom of a landfill and the water table is an important determinant of the probability that groundwater has been or may be contaminated
If the landfill has no liner but rests on impermeable bedrock, shale, or clay located above the water table, or if the depth to groundwater is great, then an unlined land-fill may pose little threat to groundwater If waste is in contact with groundwater, a surface cover cannot provide a complete remedial solution for the site The quality and quantity of native groundwater at the site are important because they control
Trang 5potential use and thus potential need for protection from contamination Therefore, the geology of the site and the lithology of geologic units between the waste and permanent groundwater are important considerations
2.3.4 g aS P roductIon
Decay of wastes and volatilization of waste components in landfills may produce sufficient toxic and explosive landfill gas to warrant gas control systems under the cover Most conventional, barrier-layer covers need an expensive gas control system because the barrier may trap the gas produced, even at low rates, and may accumu-late dangerous volumes of explosive or poisonous gas Innovative covers, such as the
ET cover, contain no barriers that might collect gas They allow landfill gas to pass through the cover soil into the atmosphere
Although gas production in a landfill can continue for a long time, high rates occur over relatively short periods, perhaps up to 10 years after the landfill becomes inactive (McBean et al 1995) Old landfills with no cover in place for 20 years or more may not need the expense of a gas collection system when covered For exam-ple, a survey of less than half of all Air Force landfills revealed that 144 landfills were both inactive for more than 20 years and not remediated in 1998–1999 (Hauser
et al 1999); they are unlikely to produce significant amounts of gas
2.3.5 S oIlS and P lantS
The availability of appropriate local soils is an important consideration in any land-fill design Conventional covers need local soils for both the foundation and the sur-face layers The soil used in an ET cover should meet the requirements for the site and support robust vegetative growth For example, ET covers may be impractical where readily available soils have inadequate water-holding capacity
The growth habits and properties of plants native to the site are important con-siderations For example, in some regions, only warm season grasses are practical for use on covers, but in others, it is possible to establish both warm and cool season grasses together on the cover The combination of warm and cool season grasses is usually more effective than single-season covers because the combination extends the time with significant plant transpiration
2.3.6 S eISmIc e nvIronment
Earthquakes are a significant threat to public safety and structures The ground shak-ing associated with earthquake activity has potential to damage landfill containment structures in many ways, including landslides on the cover, rupture of geomembrane-barrier layers, cracking of clay-geomembrane-barrier layers, breakage of conduit lines (gas control and drainage systems, electrical controls, etc.), and changes in drainage slopes Matasovic et al (1998) studied the performance of landfill covers and liners dur-ing six major earthquakes in California between 1969 and 1994 Cover performance was good to excellent at all of the landfills, with the damage limited to cracking of cover soils Within seismic hazard zones, landfill designs should be evaluated using
Trang 6site-specific seismic risk assessment criteria Richardson and Kavazanjian (1995) wrote an extensive treatment of this aspect of landfill design
2.3.7 r euSe of l andfIll a reaS
Land reuse is an important consideration in landfill cover selection and design Land-fills are warehouses for waste material built to preserve waste for an unknown length
of time; that basic requirement controls possible reuse of landfill sites All alter-nate uses for a landfill site are secondary to the primary use for waste preservation Human activity on a final landfill cover is potentially dangerous, creates the need for careful design, and may result in large cost to reduce potential injury to people Some apparently beneficial uses may conflict with primary cover purposes For example, irrigation on golf courses causes deep percolation of water below the plant-rooting zone Golf courses on landfill covers pose immediate problems because one
of the principal objectives of a landfill cover—to minimize infiltration—probably cannot be achieved under normal golf course irrigation (Hauser et al 2000)
2.4 Cover seleCtion
Previously, because federal landfill regulations contained design requirements, almost all landfill covers were barrier-type because they met the requirements of the regulators However, as stated in Section 1.4, the situation has changed and it is now practical to utilize the landfill cover technology that is most appropriate for a partic-ular site Both federal and state regulators currently support alternative technologies (ITRC 2003; US EPA 2003) An RB/PB landfill evaluation, as described in Section 2.2, allows application of the best engineering and science knowledge to select the most appropriate cover type for a particular site Where an alternative cover is appro-priate, it may provide longer and more effective containment than previously used barrier covers, and save millions of dollars in construction and maintenance cost The following 10-step process is applicable to the closure of all landfills It may be iterative, and each step may have significantly different emphasis at a particular site
1 Determine risks at the specific landfill using RB/PB methods (Section 2.2)
2 Determine site-specific performance requirements dictated by the risks at the site
3 Select the most appropriate conventional or alternative technologies
4 Elicit wide regulatory and public participation
5 Present the proposed technology to the Remedial Advisory Board and the public
6 Complete any required modeling, design criteria, and feasibility testing
7 Conduct peer reviews of the decision process and remediation design
8 Formally document the selection of the technologies in the record of deci-sion document (ROD)
9 Complete the design and monitoring plan
10 Construct all of the remediation components and gather monitoring and performance data
Trang 7Dunn, R J and Singh, U P., Eds (1995) Landfill Closures Environmental Protection and
Land Recovery Geotechnical Special Publication No 53, ASCE, Reston, VA.
Gill, M D., Hauser, V L., Horin, J D., Weand, B L., and Casagrande, D J (1999) Landfill
Reme-diation Project Manager’s Handbook The Air Force Center for Environmental Excellence (AFCEE), Brooks City Base, San Antonio, TX http://www.afcee.brooks.af.mil/products/ techtrans/landfillcovers/LandfillProtocols.asp (accessed March 14, 2008).
Hauser, V L., Gimon, D M., Hadden, D E., and Weand, B L (1999) Survey of Air Force
Landfills: Their Characteristics, and Remediation Strategies The Air Force Center for Environmental Excellence (AFCEE), Brooks City Base, San Antonio, TX http://www afcee.brooks.af.mil/products/techtrans/landfillcovers/LandfillProtocols.asp (accessed March 14, 2008).
Hauser, V L., Gimon, D M., and Jackson, D R (2000) Golf Courses on Air Force
Land-fills The Air Force Center for Environmental Excellence (AFCEE), Brooks City Base, San Antonio, TX http://www.afcee.brooks.af.mil/products/techtrans/landfillcovers/ LandfillProtocols.asp (accessed March 14, 2008).
Hicks, J., Downey, D., Pohland, F., and McCray, J (2002) Impact of landfill closure designs
on long-term natural attenuation of chlorinated hydrocarbons Parsons Corporation,
1700 Broadway, Suite 900 Denver, CO 80290 (Final report to, Environmental Security Technology Certification Program, Arlington, VA, contract no DACA72-00-C-0013.) Also available at: http://www.afcee.brooks.af.mil/products/techtrans/landfillcovers/ LandfillProtocols.asp (accessed March 14, 2008).
ITRC (2003) Technical and Regulatory Guidance for Design, Installation, and
Monitor-ing of Alternative Final Landfill Covers Interstate Technology & Regulatory Council,
444 Capitol St., NW, Suite 445, Washington, DC 20001 Also available at: http://www itrcweb.org/homepage.asp (accessed March 14, 2008).
Koerner, R M and Daniel, D E (1997) Final Covers for Solid Waste Landfills and
Aban-doned Dumps ASCE Press, Reston, VA.
McBean, E A., Rovers, F A., and Farquhar, G J (1995) Solid Waste Landfill Engineering
and Design Prentice Hall, Englewood Cliffs, NJ.
Matasovic, N., Kavazanjian, E., and Anderson, R L (1998) Performance of solid waste
landfills in earthquakes, Earthquake Spectra, 14(2), 319–334.
Richardson, G N and Kavazanjian E., Jr (1995) Seismic Design Guidance for Municipal
Solid Waste Landfill Facilities EPA/600/R-95/051, US EPA, Cincinnati, OH.
Sharma, H D and Anirban, D (2007) Municipal solid waste landfill settlement: Postclosure
perspectives, J Geotech Geoenviron Eng., 133(6), 619–629.
Suter, G W., Luxmoore, R J., and Smith, E D (1993) Compacted soil barriers at abandoned
landfill sites are likely to fail in the long term, J Environ Quality, 22(2), 217–226.
US EPA (1991) Design and Construction of RCRA/CERCLA Final Cover EPA/625/4-91/025,
Office of Research and Development, US EPA, Washington, DC.
US EPA (2003) Evapotranspiration Landfill Cover Systems Fact Sheet EPA 542-F-03-015,
Office of Solid Waste and Emergency Response, Cincinnati, OH.
Weand, B L., Horin, J D., Hauser, V L., et al (1999) Landfill covers for use at Air Force installations The Air Force Center for Environmental Excellence (AFCEE), Brooks City Base, San Antonio, TX http://www.afcee.brooks.af.mil/products/techtrans/land-fillcovers/LandfillProtocols.asp (accessed March 14, 2008).